PRAIRIE DU SAC - Coats were coming off and a few brows mopped as Chad Zeman stood talking to a group of about 60 agency and industry visitors outside the open doors of his wood-drying kiln.

Zeman is president of Green Thumb Farms, a firewood and mulch business near Prairie du Sac in Sauk County. On Feb. 25 he hosted the on-site portion of a training workshop for heat treatment of firewood.

The kiln, with doors shut and a 650,000 Btu boiler running at full tilt, will get the internal temperature of 13.5 to 14 cords of cut and split firewood to at least 160 degrees in about six to eight hours.

The garage-size kiln meets federal certification standards for heat treating commercial firewood before it can be sold into certain restricted states.

Once the required heat is achieved, excess moisture is vented to the outside, and the kiln runs another three to four days to dry the wood to optimum fireplace moisture of less than 20 percent.

Zeman has been operating a firewood business since high school, when he delivered small loads to Madison as winter income to supplement a summer lawn and landscaping business he owned.

The Green Thumb wood delivery area, processor, kiln and packaging building are on the edge of a 76-acre fruit farm and nursery that Zeman and his father, Rick, bought seven years ago. The farm still supports a fall pumpkin patch with horse-drawn wagon rides and petting zoo, but the heart of the operation for Zeman is the kiln.

"We chose this system because we're going from log form to going out the door in less than 10 days," he said.

At its peak in the fall, the business employs six to seven people who operate the processor, load and unload the kiln and stretch-wrap bundles of wood for shipment to as far away as Ohio, New York and North Carolina.

"Most of it is for home heating, but it's a novelty for fireplace people," Zeman said.

Before the kiln was installed two years ago, Zeman was taking up to a year to air-dry firewood for local use. He also shipped 1,900 to 2,300 cords a year of green, split firewood in bulk loads to Chicago. With the advent of emerald ash borers in the Midwest, shipping green wood is a thing of the past.

Zeman's goal was to reduce inventory by getting finished firewood out the door faster.

Loads of pulpwood and occasionally saw logs are delivered from a four-county area. Wood is sorted, cut to firewood length, split in eighths and piled in a staging area for the kiln.

Zeman designed custom bins to hold the wood in the kiln.

"They're half-cord, and we got them designed with a tailgate like a dump truck so we can just hook onto them and dump them," he said.

Electronic sensors in the kiln feed heat information to a computer port that can be accessed for continuous readings during the heat-treating and drying process.

A year ago, the original heat exchanger in the kiln was retrofitted with additional fins to get more heat transferred to the inside air. Baffles were added to direct the air more efficiently around the bins, and 8-gallon pumps were replaced with 18-gallon-per-minute pumps off of the boiler.

Zeman said the fins, baffles and bigger pumps increase the system's efficiency by about 100,000 Btu.

The project was directed by Xiping Wang, a senior research associate at the University of Minnesota-Duluth Natural Resources Research Institute, stationed at the U.S. Department of Agriculture Forest Products Laboratory in Madison.

The power plant is an outdoor wood-fired boiler.

"In the winter, when it was 20 to 30 below, about every four to five hours we were filling it, and it holds a lot of wood," Zeman said.

He burns anything he can get for free, including butt ends, cottonwood and logs from urban timber sources. The first logs to go in must be dry to get the temperature up fast, and then greener wood can be added to keep a slow fire for drying.

Zeman said he has about $90,000 invested in the system, including the baskets. He hasn't calculated the payback period but said the boiler has the capacity to run three to four buildings. He already has plans to add another kiln.

He calculated his cost at less than $20 a cord to run the system.

Steam coming off the drying firewood could provide a heat source for other parts of the operation.

"We've got a friend of ours who is thinking about how we can use that steam to heat another building to pre-heat the wood in the winter to get it up to 80 or 90 degrees instead of below zero," Zeman said. "We have a lot of engineering to do on that."

Terry Mace, one of the instructors for the heat treatment workshop, said kiln operations such as Zeman's may be a necessity in the future. Emerald ash borers are in Wisconsin, and the insects are too aggressive to think they won't spread to the whole state.

"What I'm looking at is getting the companies to where they know what they need to do," Mace said. "They can get prepared so when they get quarantined they can meet the standard so they can stay in business."

Zeman's is one of only about four dry wood kilns of its type in Wisconsin.

Sara Bredesen can be reached at 715-360-7253 or stbrede@gmail.com.